The demand for weight reduction in the automotive industry has led to the development and implementation of lightweight materials or components, and related manufacturing processes and tools. The demand for weight reduction is especially driven by the goal of reduction of CO2 emissions. The growing concern for occupant safety also leads to the adoption of materials which improve the integrity of the vehicle during a crash while also improving the energy absorption.
A process known as Hot Forming Die Quenching (HFDQ) uses boron steel sheets to create stamped components with Ultra High Strength Steel (UHSS) properties, with tensile strengths of e.g. 1.500 MPa or 2000 MPa or even more. The increase in strength allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components.
There are several known Ultra High Strength steels (UHSS) for hot stamping and hardening. The blank may be made e.g. of a boron steel, coated or uncoated, such as Usibor® (22MnB5) commercially available from ArcelorMittal.
Typical vehicle components that may be manufactured using the HFDQ process include: door beams, bumper beams, cross/side members, NB pillar reinforcements, and waist rail reinforcements.
Hot forming of boron steels is becoming increasingly popular in the automotive industry due to their excellent strength and formability. Many structural components that were traditionally cold formed from mild steel are thus being replaced with hot formed equivalents that offer a significant increase in strength. This allows for reductions in material thickness (and thus weight) while maintaining the same strength. However, hot formed components offer very low levels of ductility and energy absorption in the as-formed condition.
In order to improve the ductility and energy absorption in specific areas of a component, it is known to introduce softer regions within the same component. This improves ductility locally while maintaining the required high strength overall. By locally tailoring the microstructure and mechanical properties of certain structural components such that they comprise regions with very high strength (very hard) and regions with increased ductility (softer), it may be possible to improve their overall energy absorption and maintain their structural integrity during a crash situation and also reduce their overall weight. Such soft zones may also advantageously change the kinematic behaviour in case of a collapse of a component under an impact.
Known methods of creating regions with increased ductility (“softzones” or “soft zones”) in vehicle structural components involve the provision of tools comprising a pair of complementary upper and lower die units, each of the units having separate die elements (steel blocks). A blank to be hot formed is previously heated to a predetermined temperature e.g. austenization temperature or higher by, for example, a furnace system so as to decrease the strength i.e. to facilitate the hot stamping process.
The die elements may be designed to work at different temperatures, in order to have different cooling rates in different zones of the part being formed during the quenching process, and thereby resulting in different material properties in the final product e.g. soft areas. E.g. one die element may be cooled in order to quench the corresponding area of the component being manufactured at high cooling rates and by reducing the temperature of the component rapidly. Another neighbouring die element may be heated in order to ensure that the corresponding portion of the component being manufactured cools down at a lower cooling rate, and thus remaining at higher temperatures than the rest of the component when it leaves the die.
The use of multistep press systems for manufacturing hot formed elements is known. The multistep press systems may comprise a plurality of tools configured to perform different operations on blanks simultaneously. With such arrangements, a plurality of blanks undergo different manufacturing processes simultaneously during one stroke using the tools forming the multistep press systems, thus the performance of the system may be increased.
A multistep press system may include a conveyor or a transferring device which transfers the heated blank to a press tool which is configured to press the blank. Additionally, a furnace system that heats and softens the blank to be hot formed may be provided upstream from the multistep press machine. Furthermore, a separate laser process step or a separate cutting tool may also be provided, wherein the stamped blanks are discharged from the press system and are transferred and located into the laser process step or in the separate cutting tool in order to be manufactured e.g. cut and/or trimmed and/or pierced and/or punched.
Generally, in such systems, an external pre-cooling tool is used in order to previously cool down the blank to be hot formed. Once the blank is cooled down, it is transferred from the external pre-cooling tool to the multistep press apparatus or system.
WO2011115539 describes a contact-cooling press provided between a furnace and a press-hardening press. Preselected parts of a blank (18) are contact-cooled such that corresponding parts of the finished product are softer and display a higher yield point.
The present disclosure seeks to provide improvements in multistep systems configured to create soft zones and methods.